Right now you can only use standard enemies with their standard decks in custom scenarios. It would give modders a lot of new options to create interesting scenarios if they could change the decks or even create new ones.
This has been suggested a lot by the map making community in the past (and very recently) - especially now that some wants to be able to make their own league suggestions. I believe there's no real interface that would make this work, since I think the game loads monster deck lists from the server. If there was a simple way to load it manually via the custom map tool, I'm sure we would have had the opportunity already.
